The primary goal of MM art has not really been to convey a sense of evocation or to build atmosphere around the creatures, but simply to show what the monster looks like, hopefully depicted in a way that it can easily be described to the players. The MM is meant to catalogue foes and provide description and mechanics, not to inspire the encounters themselves. That was never the intention that artists were working under when they were drawing, and so they didn't shape their art to reflect it. They are technical pieces of art - more utilitarian, which suits something called a "Manual". When the pendulum swings the other direction, you get the art-punk scribbles that are Fire on the Velvet Horizon and its ilk - inspirational as hell, but good luck using it at the table without a bunch of prep.
Your vampire picture comparison is a good example of that. Yes, the second picture looks like more like a vampire and evokes a horror vibe in you, the DM reader, but all the players will ever get is your description of it. They don't get to see either picture.
I can however more easily describe that 2e vampire in a gameable way to my players in a session: "a fellow of noble bearing draped in a violet cloak, he steadies himself with a walking stick and squints at you from beneath a judgmental brow". Hell, it's just a stand-in picture to show that vampires can look like a normal person (that's their deal, hiding in plain sight, feasting on the innocent). Is it a vampire or a man? Not sure, but it certainly explains why he can walk around with normal society, and leaves room for my players to dance with the encounter.
The 1e description would be "a pale man with a pronounced widows' peak hisses at you from behind his tall-collared cloak, fangs bared". Players hear that and go "ah, a vampire!". If I describe that to my players, they just attack on sight. No wonder, no mystery, no evocative scene - just "here's what you see", and in this case, it's obviously a vampire, and so obviously not surprising when it lunges at some unwary player's neck, and likewise not surprising when the players immediately go for their silvered weapons and garlic. Everyone knows what vampires are and everyone knows what the stereotypical vampire looks like; the art is superfluous, and it's job at that point is to exist because all the other entries have pictures.
The other 2e vampire? Not so much. I see a full picture of a person I can describe to the players, but also I understand that vampires are meant to be indistinguishable from normal people, and so can go ahead to further describe other vampires, or to change the look of this one that doesn't end up breaking the intent of the creature in the game. Much more useful art for me to have, in that regard.
I suppose the difference between you and I are how we employ our MM - I use it as a reference Manual, and you are using it as an idea-generator.
